Infrared radiometer apparatus for detecting clear-air turbulence from an aircraft

ABSTRACT

A radiometer apparatus suitable for use on a supersonic aircraft for clear-air turbulence detection includes means for operating at four selected wavelengths within an atmospheric absorption band, for example the 15 Mu band of carbon dioxide. At any particular height of the aircraft each of a set of filters used to select the four selected wavelengths has a weighting function which depends on the effective absorption coefficient of the atmosphere at that wavelength and each indicates which part of the atmosphere ahead provides a significant contribution to the radiation measured by the apparatus. At longer wavelengths the atmosphere is opaque and the radiometer apparatus receives radiation from immediately in front of the aircraft. By generating suitable functions from the outputs of radiation passed by two or more of the filters it is possible to detect turbulence of the type looked for.

United States Patent 1151 3,641,345 Coackley et al. Feb. 8, 1972 [54]INFRARED RADIOMETER APPARATUS 3,465,339 9/1969 Marner .Q ..250/83.3 1-1x FOR DETECTING .AIR 3,551,678 12/1970 Mitchell ..250/83.& 11

[ Inventors! Robert y; Michael Leslie Reynolds, AssistantExaminer-Morton J. Frome both of y; Clive Douglas Attorney-Cushr'nan,Darby & Cushman Rodgers, Oxford, all of England [73] Assignee: NationalResearch Development Corpora- [57] ABSTRACT London- England A radiometerapparatus suitable for use on a supersonic air- 2 Filed; Man 5 9 craftfor clear-air turbulence detection includes means for operating at fourselected wavelengths within an atmospheric [211 P 16,922 absorptionband, for example the 15 hand of carbon dioxide.- At any particularheight of the aircraft each of a set of filters 30 Foreign Applicationpriority m used to select the four selected wavelengths has a weightingfunction which depends'on the effective absorption coeffi- Mar. 6, 1969Great Brltaln ..l 1,868/69 ciem ofthe atmosphere at that wavelength andeach indicates which part of the atmosphere ahead provides a significant(gl. contribution to h di tion measured by the apparatus. At [58] s i 673/355 R longer wavelengths the atmosphere is opaque and the radiometerapparatus receives radiation from immediately in front of the aircraft.By generating suitable functions from the [56] References cued outputsof radiation passed by two or more of the filters it is D STATES PATENTSpossible to detect turbulence of the type looked for. 3,475,963 11/1969Astheimer ..250/83.3 H X 23 Claims, 9 Drawing Figures 7Q 77 )5 MOTOR ATIMING LOGIC LOGIC GENERATOR 69 7o RESET L, 57\ 53 INTEGRATO I FILTER 65I MOTOR 45 GA\N 6g ETECTOR AMPLIFIER QE$ CHANGE PS 59 PLIFIER CHOPPERc|-|oPPeri PHASE MOTOR MOTOR SHIFT TURBULENCE FROM AN AIRCRAFT PrimaryExaminer-James W. Lawrence PATENTEHFEB 3 1972 SHEET 1 BF 7 n 5 I 6 S 5.umDbxmwmvzuk E4 I00 I20 I40 RANGE, km

FIG.

FIG.

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SHEET 2 BF 7 R Q P O J I 1 l l 'v I I I FIG. 3. W E NUMBERS (cM") QBRANCH GENERALISEDT ABSORPTION I COEFFECIENT I 1 I 1 I 70 680 690 7007:0 720 730 74o WAVE NUMBERS cm FIG. 4.

% mmieom 8 I972 3,641.85

i sum 3 OF 7 40 8O DISTANCE rkm OF STEP FROM RADlOMETER PAIENTEBFEB81972 SHEET 5 OF 7 A5 zoFuzE M623.

8 0 DISTANCE (r k m) OF STEP FROM RADIOMETER FIG. 7.

WW w ALE m M VE .w n/ b M [Z 4 My ww p T; HM 5A 00 R X PATENIEBFEB m23.61345 SHEET 5 BF 7 FIG. 8.

INFRARED RADIOMETER APPARATUS FOR DETECTING CLEAR-AIR TURBULENCE FROM ANAIRCRAFT The present invention relates to radiometer apparatus.

Atmospheric turbulence creates a continual hazard toair travel, both interms -of passenger discomfort and possible damage to aircraft,resulting in extreme cases to loss of machines and life. A particulartype of turbulence, associated with clear air, is singularly dangerousas its location and presence is not predictable by normal meteorologicalprocedure.

' A considerable weight of evidence has been accumulated in theliterature, showing that most cases of clear-air turbulence areassociated with temperature gradients in the atmosphere. It is thoughtthata threshold temperature change of 0.l per kilometer existingoverseveral kilometers is associated with this phenomenon.

It is an object of the present invention to provide a radiometer capableof detecting such a temperature change remotely.

According to the present invention there isprovided .a radiometerincluding a detector sensitive to incident radiation from the atmospherehaving only wavelengths in a first band within an absorption band of agiven component of the atmosphere, a detector sensitive to incidentradiation from'the atmosphere having only wavelengths in a second bandwithin the absorption band of the given component of the atmosphere,wherein the second band is such as to have a generalized absorptioncoefficient of a different magnitude from that of the first band, and anindicating device responsive to the difference in magnitude between thesignal of the detected radiation in the first band and the signal ofdetected radiation in the second band.

The radiometer may further include a detector sensitive to incidentradiation having wavelengths only in a third band within the absorptionwaveband wherein the third band is such as to have a generalizedabsorption coefficient which is large compared with that of the secondband, and an indicating device may be responsive to the quotient of thesaid difference and the detected radiation in the third band.

The detector sensitive to incident radiation having a wavelength in thefirst band, the detector sensitive to incident radiation having awavelength in the second band and the detector sensitive to incidentradiationhaving a wavelength in the third band may all be the samedetector having alternative bandpass filters for the three respectivepass bands. In fact a set of filters having relative absorptioncoefficients in ascending powers of two may provide for use of theradiometer at different pressure, and hence at different heights in theatmosphere. 7

An embodiment of the invention will be described by way of example withreference to the drawings accompanying this specification in which:

FIG. 1 is a graph of air temperature plottedagainst'distancc in asituation where clear turbulence was observed;

FIG. 2 is a diagram of a model temperature profile of clearairturbulence used for calculating radiometer performance FIG. 3 is a graphof carbon dioxide absorption plotted against wave numbers;

FIG. 4 is a graph of the generalized absorption coefficient of carbondioxide integrated over intervals of five wave numbers;

FIG. 5 is a graph of responses of a radiometer instrument channel to amodel temperature step of the sort illustrated in FIG. 2;

FIG. 6 is a graph of difference responses plotted against distance froma radiometer;

FIG. 7 is a graph of range function plotted against distance from aradiometer;

FIG. 8 is a cross-sectionaldiagram of a radiometer.embodying theinvention; and

FIG. 9 is a block schematic diagram of a possible electronics system forthe radiometer apparatus.

FIG. I is a graph of air temperature plotted against distance in asituationwhcre clear-air turbulence was observed. The air 7 wheretaining its higher temperature for some further 50 kilometers andfalling towards its ambient value again in a distance of 40 kilometers.Moderate to severe clear-air turbulence was experienced'in a range,indicated by the reference CAT in FIG. 1, some kilometers in length,during this temperature change. A 4 C. temperature rise spread overseveral tens of kilometers may well be typical of clear-air turbulence.

FIG. 2 is a diagram of a model temperature'profile of clearairturbulence used for calculating radiometer performance. The range of thetemperature step from the radiometer is measured at the very beginningof the temperature rise, as soon as the local temperature rises-aboveambient. The initial temperature gradient is assumed to be 4 C.distributed linearly over a 20-kilometer range distance,.and thereafterthe temperature is assumed to be constant and 4 C. above ambient for afurther 20 kilometers, and finally the temperature is. assumed to falllinearly down to ambient over a further distance of 20 kilometers. Atemperature step with a profile like this should be detectable by aninfrared radiometer. In other words, provided there exists a temperaturestep of this form then it should be possible to use an infraredradiometerto determine the existence and range of the temperature stepprovided thatthe radiometer is receptiveto a wavelength in the infraredportion of the electromagnetic spectrum where there is radiation fromthe atmosphere. 7

If the absorptivity of a slab of the atmosphere as a function ofwavenumber v is 01(11), then, by Kirchoffs law, it will emit radiationwith power a( v)B(a,0), where B(a,0) is the power that would be emittedby a black body at the temperature of the slab, 0. If the slab is ofthickness dx at distance x from a radiometer and T(v,x) is thetransmission function of the atmosphere, then and the power reaching theradiometer from the slab is aBT= B (ix.

rlx I We may integrate over x and 1/, including the spectral response ofthe radiometer, f(v), to obtain the total power, S, reaching theradiometer:

S=IIf(1 )B(v,x) )dxd u. .3",

(B can be expressed as a function of x since 0 isa function of x) If wemake'the assumption that f(v) is sharply peaked at v=v sothat B(v) doesnot differ appreciably from B(v,,) over the wave number range, 1/,forwhichflv) is significant, the integral may be rewritten as so... Kwdx. (1)

a K(x) =L gv till. 2 The signal'S is seen to be a weighted sum ofblack-body powers with weighting function K(x).

We could use this expression to calculate S if we knew 'T(v,x) and 6(x)and hence K(x) and B(v,x), but the inverse problem is of more interest.That is, given radiometer signals for several spectral regions, todeduce either 0 (x) if T is known or to deduce T if 0(x) is known. Ofcourse, it is only possible to deduce the form of a continuous functionfrom-the values of a finite number of signals describing the function interms of parameters. In general, the more spectral intervals we measure,the more parameters we can determine.

The radiometer described is receptiveto the IS-micron car- ;bon dioxideabsorption band which is reasonably close to the peak of the black-bodycurve-at temperatures in-theupper at- K.). Carbon dioxide has asufficiently constant mixing ratio for the transmission to be calculatedand the temperature structure deduced from the radiometer readings.

FIG. 3 is a graph of carbon dioxide absorption plotted against wavenumbers. The carbon dioxide spectrum consists of a narrow region of verystrong absorption at a wave number of 668 cm? (the Q branch, soindicated in FIG. 3) corresponding to the frequency of the bending modeof vibration of the molecule, together with rotational sidebands (the Pand R branches, so indicated in FIG. 3). The P and R branches consist ofalmost evenly spaced absorption lines, and, as these are about 1.5 cmflapart, and a typical radiometer bandwidth is emf, it is the averageabsorption of such a band that is of interest.

FIG. 4 is a graph of the generalized absorption coefficient of carbondioxide integrated over intervals of five wave numbers.

The generalized absorption coefi'lcient B has units of km and is givenby where S line strength in cm.

a= line width in cm? 1. mixing ratio of carbon dioxide (mass/mass)pressure in millibars 8 line spacing in cm. and the lines referred toare the absorption lines of FIG. 1. The absorption line widths or varywith pressure and the result is that the absorption coefficient B isproportional to the square of the pressure. FIG. 4 shows its variationwith wavelength and temperature. From having a high value (about 12 km.)near the Q branch at 670 cm. the value of B falls to below 1 km. at some717 cm. near a second Q branch at 720 cm. The fall is more or lesslogarithmic between 683 cm." (where it has values around 11 at 300 K.and 10.5 at 200 K.) and 713 cm. (where it has values around 0.18 km. at300 K. and 0.12 km? at 200 K.). I

The radiometer embodying the invention is a multichannel infraredradiometer designed to measure the size of and the approximate range toan atmospheric temperature discontinuity similar to that illustrated inFIG. 2 from an aircraft flying between 30,000 and 60,000 feet. Theradiometer operates at four selected wave lengths between 668 cm. and720 cm. of one edge of the carbon dioxide absorption band. Theinstrument has to be so made that it can accommodate the different totalabsorption values which occur as the atmospheric pressure varies fromabout 300 mb. at 30,000 feet to 75 mb. at 60,000 feet. In fact theinstrument is correctly calibrated at four specific heights, namely 212mb. (37,000 feet), 150 mb. (45,000 feet), 106 mb. (52,000 feet) and 75mb. (60,000 feet) by a suitable choice of infrared filters. Each set offilters is used at intermediate heights below the appropriate correctlycalibrated level.

At any particular height each filter has associated with it anatmospheric weighting function. This weighting function depends on theeffective absorption coefficient of the atmosphere at that wavelength(itself not a simple function) and indicates which part of theatmosphere ahead provides a significant contribution to the radiationmeasured by the instrument. Thus at the longer wavelengths where theatmosphere is very opaque the radiometer receives radiation fromimmediately in front of the aircraft so measuring the local temperature,whereas at the less attenuating wavelengths the radiation comes alsofrom more remote sections of the line of sight. Because of the curvatureof the earth, a horizontal line of sight is in fact a tangential pathand will eventually emerge into space from an increasingly rarifiedatmosphere. Care must therefore be taken when choosing the filter forleast attenuating wavelength that at this frequency the radiometercannot receive radiation from outside the atmosphere. If it does so,that channel will give a false reading and moreover will be verysusceptible to pitch errors since pitch angle significantly affects theeffective distance to space.

Because of the increased rarity of the atmosphere as one ascends, thefilter having the least absorption would eventually allow penetration byradiation from space. It is therefore necessary to discard this filterin favor of one at a different wavelength. By switching, carefullychoosing the weighting functions used and the changeover height, it ispossible to manage with only seven filters, used in the successiveheight bands. A suitable set of seven filters would have relativeabsorption coefficients at fixed pressure of 1, 2, 4, 8, 16, 32, andabout' 100. They would be used according to the height of the aircraftas shown in the following Table 1:

Thus at any height range four filters will be in operation. These fourfilters will have different relative absorption coefficients and willtherefore be sensitive to temperatures at different distances from theaircraft. For example the most absorbent filter (that with a relativeabsorption coefficient of about will be sensitive only to thetemperature immediately outside the radiometer. Therefore, by suitablecombination of the outputs of four detectors having the four differentrelative absorption coefficients, or one detector having the fourfilters applied to it in turn, it should be possible to detect atemperature step of the sort illustrated in FIG. 2.

The frequencies transmitted by the seven filters are set out in thefollowing Table 2:

relative absorption absolute absorption atmosphere coefficientcoefi'lcient B at 220 K. at 1 atmosphere width of filter to transmissionpoints frequency cm.

This filter must cut off at 717.5 cm. to avoid the Q branch at 720 cm.

Given the filters specified in Table 2 above, the responses of the fourinstrument channels to a temperature step such as that illustrated inFIG. 2 at the four design heights, i.e., the heights at which thefilters are correctly calibrated, namely 37,000 feet, 45,000 feet,52,000 feet and 60,000 feet will be as shown in FIG. 5. The response ineach channel diminishes rapidly with distance, but channel 1 is stillsensibly responsive to a temperature step 200 km. from the radiometer.

In order to detect a temperature step of the type illustrated in FIG. 2a range function R is generated, where 1 z)/( ;r' 4) and S S S and S.are the outputs of the four instrument channels of the radiometer inascending order of absorption coefficient. The range function will bevery sensitive to temperature steps some hundreds of kilometers in frontof the aircraft.

FIG. 6 is a graph of difference responses plotted against distances fromthe radiometer. A curve S,S illustrates the difference between theoutputs of the instrument channels S, and S and a curve S S illustratesthe difference between the outputs of the instrument channels 8;, and SThe numerator S -S of the range function has a value at zero rangeequivalent to an effective black-body temperature difference of around-0.05 times the height of the temperature step and this value increasesto zero at about 10 kilometers range and to a maximum of about +0.04 at60 kilometers range, falling to about +0.01 at 200 kilometers range. Thedenominator S,-S of the range function has a value at zero rangeequivalent to an effective difference black-body temperature of around0.2 times the height of the temperature step and this value increasesand a maximum of about 0.27 at some 5 kilometers range. Thereafter thevalue S is sensibly zero and the value follows the value 8,, fallingasymptotically to zero.

FIG. 7 is a graph of range function plotted against distance from aradiometer: this is a function which increases rapidly with range aswill be readily apparent from consideration of the graphs of FIG. 6.

A likely way for the radiometer to be used would be as follows. Innormal flight the radiometer would be used in a search mode, notscanning, but receiving radiation from a horizontal path ahead of theaircraft. In this mode the radiometer would be responsive only to thefirst channel and thus would have the maximum capability of detecting atemperature rise ahead of the aircraft. Once such a temperature rise isdetected, channel 2 also would be monitored and the signal S,Sgenerated. However, this signal has a higher amount of noise than eitherthe signal S, or the signal S by themselves and so it may be necessaryto use multilook correlation to limit the false alarm rate. A total oflooks, taken in sequence should occupy less than a minute, during whichtime the signal should be increasing. An estimate of the temperaturerise can be made and if necessary a preliminary warning given to thepilot At such a time, the channel 3 also would be monitored and thesignal (S,S )/S generated. Such a signal is identical with thatillustrated in FIG. 7 down to a range of some 10 kilometers. Finally,the range function R may be generated and used to give an exact rangefor the clear-air turbulence.

FIG. 8 is a cross-sectional diagram of a radiometer embodying theinvention. A housing 11 is fixed in an aircraft, the skin of which isshown at 13, by mountings 15. A turret 17 is free to rotate about avertical axis within the housing 11 on an upper bearing 19 and a lowerbearing 21. The upper bearing 19 has a central opening 23 to allowinfrared radiation to pass through it. An orifice 25 in the forwardportion of the housing 11 admits radiation to the inside of the turret17 via a window 27 transparent to infrared radiation of the frequenciesof interest.

A mirror 29 is mounted in the turret 17 and is free to rotate about anaxis perpendicular to the axis of rotation of the turret l7 and parallelto the window 27. The mirror 29 is generally at 45 to the axis ofrotation of the turret 17, and is thereby adapted to deflect radiationincident on the window 27 through the opening 23. Therefore to betransmitted through the opening 23 radiation must be received along asight line 31 which varies in space according to the attitude of theaircraft and which varies relative to the axis of rotation of the turret17 according to the angle of the mirror 29 relative to the axis ofrotation of the turret 17 It is therefore possible to stabilize thesight line 31 in direction in space by controlling the angle between themirror 29 and the axis of rotation of the turret 17. This is achieved bya gyroscope 33 sensitive to pitching of the aircraft and driving a trainof gears 35 which in turn control the angle between the mirror 29 andthe axis of rotation of the turret 17.

Similarly the sight line 31 is controlled in azimuth by the rotation ofthe turret 17 within the housing 11 under the control of a secondgyroscope 37 sensitive to yaw in the aircraft. The gyroscope 37 drivesthe turret 17 on its axis relative to the housing 11 via a train ofgears 39. This arrangement may also be used for scanning the sight line31 in azimuth, whereby radiometer readings may be taken to the left andto the right of the course of the aircraft: if there is a strongpossibility of clear-air turbulence ahead of the aircraft then it isconvenient to be able to detect whether the situation would be improvedby altering course, and readings of up to 45 on either side of thecourse of the aircraft are very useful.

Radiation incident on the radiometer from distant parts of theatmosphere is brought to a focus at the central opening 23 either by thewindow 27 being a convex lens or by the mirror 29 being a concavemirror. The central opening 23 contains a field stop 40 and field lens41 and beyond the field stop 40 and field lens 41 is a projection lens43 which is arranged for throwing the radiation on a detector 45 mountedinside a cryostat unit 47. A chopper disk 49, driven by a chopper motor51, is interposed between the mirror 29 and the central opening 23. Afilter disk 53 contains a plurality of filters 55,

which are interposed between the field stop 40 and field lens 41 and theprojection lens 43. The filter disk is controlled by a filter motor 57.

The window 27 and the lenses 41 and 43 are made of infrared wide-bandtransmitting material, for example germanium. Similarly the surface ofthe mirror 29 is made of infrared wide-band reflecting material, forexample the mirror 29 may be made of aluminum, nickel plated and coatedwith gold.

The entrance aperture of the radiometer is defined by the window 27 andthe instrument field of view is defined by the mechanical field stop 40,which is in the focal plane of the window 27/mirror 29 optical systemand the size of which is defined by the required field of view and thefocal length of the window 27/mirror 29 system. The function of thefield lens 41 is toproject a real image of the window 27 on theprojection lens 43. The projection lens 43 then projects a real image ofthe field stop 40 on the detector 45.

The detector 45 may be a mercury cadmium telluride detector cooled to 77K. by the cryostat 47 or a copper-doped germanium detector cooled toliquid helium temperatures by the cryostat 47.

FIG. 9 is a block schematic diagram of a possible electronics system forthe radiometer apparatus. FIG. 9 shows the detector 45 being illuminatedalong a light path 59 via the chopper disk 49, driven by the choppermotor 51, and the filter disk 53, controlled by the filter motor 57. Theoutput of the detector. 45 is fed through an amplifier 61, a synchronousfilter 63, a gain-change amplifier 65 and a phase-sensitive detector 67to a resettable integrator 69. The chopper motor 51 is driven by achopper drive unit 71, which also supplies, via a phase shift unit 73the synchronous filter 63 and the phase-sensitive detector 67. A resetpulse generator 75 is connected to reset the resettable integrator 69.The filter motor 57 is controlled by a timing logic unit 77 via a motorlogic unit 79. The timing logic unit 77 also controls the reset pulsegenerator 75.

The action of the circuit is as follows. Light falling on the detector45 via the path 59 is chopped by the chopper disk 49 under the controlof the chopper motor 51 and is filtered by the appropriate filter in thefilter disk 53 so that only the wavelength called for will fall on thedetector 45. The lowlevel signal from the detector 45 will be fed viathe amplifier 61 (which includes a low-noise pre-amplifier and a mainamplifier) to the synchronous filter 63. The synchronous filter 63consists of a series-connected resistor followed by a firstparallel-connected capacitor in series with a first electronic switchand a second parallel-connected capacitor in series with a secondelectronic switch. The first electronic switch is switched in phase withthe output of the phase shift unit 73 and the second electronic switchis switched in antiphase with the phase shift unit 73. The function ofthe unit is to limit the noise bandwidth before further amplification.The output of the synchronous filter 63 is applied to the amplifier 65and thence to the phase-sensitive detector 67, where it is synchronouslyrectified and applied to the integrator 69. After, say, one second theoutput of the integrator 69 will be sampled via its output channel 70and it will be immediately reset by a pulse from the reset pulsegenerator 75. The timing logic unit 77 will control both the reset pulsegenerator and the motor logic unit 79 which controls (via the filtermotor 57) the filter disk 53, and hence the wavelength received by thedetector 45. The output 70 may be recorded in any suitable manner, forexample by a tap recorder (not shown).

It is required to produce both difference and quotient functions from aset of four consecutive samples emerging from the channel 70. To thisend a second integrator (not shown) may be attached to he output of thesignal-channel main amplifier 65 and arranged so that one signal isintegrated from a zero starting level after which the signal to besubtracted from it is fed in with the opposite polarity and for the sameperiod of time; at the end of this period the signal at the output ofthe integrator is the difference between the two input signals.

Quotient functions may be derived by various means. One method is to useone signal to set the gain of an amplifier,

while the other signal is passed through it in such a way that theoutput is proportional to the ratio of the two signals. Alternativelyone signal may be used to set the amplitude of a pulse train while theother determines the width of each pulse. The area under the pulse canreadily be arranged to be proportional to the ratio of the two signals,the wanted signal being recovered by means of a DC coupled low-passfilter.

The seven filters 55 on the filter disk 53 may be arranged on the filterdisk 53 so that a set of four consecutive filters may be selected inevery case, by arranging the filters in the following order on thefilter disk 53: l, 2, 4, -:l00, 8, 16, 32. In this way the motor logic79 may be simplified, and may be arranged to switch any group of fourfilters in sequence in synchronism with the signal-sampling electronics.Simple filter-position detectors (for example reed switches) will enablethe various signals to be unambiguously identified and processed.

The various filter sequences may be selected automatically according toheight, or by means of a preset program. The filters within thesequences may be similarly selected by an automatic timer or manually.

We claim:

1. An infrared radiometer apparatus for detecting clear-air turbulencefrom an aircraft comprising:

a first bandpass filter means for passing radiation from the atmospherehaving only wavelengths in a first sub-band. within an absorption bandof a given component of the atmosphere,

a second bandpass filter means for passing radiation from the atmospherehaving only wavelengths in a second subband within said absorption band,said second sub-band having a different generalized absorptioncoefficient from said first sub-band,

a third bandpass filter means for passing radiation from the atmospherehaving only wavelengths in third sub-band within said absorptionwaveband, said third sub-band having a different generalized absorptioncoefficient from said first and said second sub-bands,

infrared detector means located in the path of radiation passing throughsaid first, second and third bandpass filter means and sensitive toradiation passing therethrough for generating first, second and thirdsignals corresponding respectively to radiation having wavelengths insaid first, second and third sub-bands, and

quotient-indicating means responsive to the magnitude of said first,said second and said third signals and indicative of a quotient functionrepresenting the difference between the magnitude of said first signaland the magnitude of said second signal, the difference being divided bythe magnitude of said third signal.

2. A radiometer apparatus as claimed in claim 1 and wherein one and thesame detector is sensitive to incident radiation in said first, saidsecond and said third sub-bands.

3. A radiometer apparatus as claimed in claim 1 and wherein said first,second and third bandpass filter means are operable with radiation fromthe atmosphere having only wavelengths in sub-band within the carbondioxide absorption waveband at and around l5 microns.

4. A radiometer apparatus as claimed in claim 1 having certain line ofsight usable on an aircraft and further comprising means for stabilizingsaid line of sight against aircraft pitching, and means for scanning inazimuth said line of sight.

5. A radiometer apparatus as claimed in claim 1 and further comprising afirst housing member for housing the optics of said radiometerapparatus,

a second housing member rotatable within said first housing member aboutan axis perpendicular to said line of sight,

a mirror within said second housing member for deflecting incomingradiation towards said filter means,

a first gyroscopic means, sensitive to aircraft pitching, forcontrolling the angle between said mirror and said axis about which saidsecond housing member is rotatable, and

a second gyroscopic means'for rotating said second housing member andscanning in azimuth said line of sight.

6. A radiometer apparatus as claimed in claim 1 and further comprising:

a fourth bandpass filter means for passing radiation from the atmospherehaving only wavelengths in a fourth sub-band within said absorptionwaveband, said fourth subband having a different generalized absorptioncoefficient from that of said first, said second and said thirdsub-bands,

said detector means being sensitive to radiation passing in said fourthsub-band for generating a fourth signal corresponding to radiation insaid fourth sub-band,

and wherein said quotient-indicating means is responsive also to themagnitude of said fourth signal corresponding to radiation havingwavelengths in said fourth sub-band and indicative also of a quotientfunction representing the quotient of the difference between themagnitude of said first signal and said second signal and the differencebetween the magnitude of said third signal and said fourth signal.

7. A radiometer apparatus as claimed in claim 6 and wherein one and thesame detector is sensitive to incident radiation in said first, saidsecond and said third sub-bands,

8. A radiometer apparatus as claimed in claim 6 and wherein one and thesame detector is sensitive to incident radiation in said first, second,third and fourth sub-bands.

9. A radiometer apparatus as claimed in claim 6 and wherein said first,second, third and fourth bandpass filter means are operable withradiation from the atmosphere having only wavelengths in sub-band withinthe carbon dioxide absorption waveband at and around 15 microns.

10. A radiometer apparatus as claimed in claim 6 and having a certainline of sight usable on an aircraft and further comprising: means forstabilizing said line of sight against aircraft pitching and means forscanning in azimuth said line of sight.

11. A radiometer apparatus as claimed in claim 10 and further comprisinga first housing member for housing the optics of said radiometerapparatus,

a second housing member rotatable within said first housing member aboutan axis perpendicular to said line of sight,

a mirror within said second housing member for deflecting incomingradiation towards said filter means,

a first gyroscopic means, sensitive to aircraft pitching, forcontrolling the angle between said mirror and said axis about which saidsecond housing member is rotatable, and

a second gyroscopic means for rotating said second housing member andscanning in azimuth said line of sight.

[2. A radiometer apparatus as claimed in claim 6 and further comprising:

a further set of bandpass filter means in addition to said first,second, third and fourth bandpass filter means,

a filter housing member for housing said first, second, third and fourthbandpass filter means and said further set of bandpass filter means, and

means for rotating said filter housing member and for removing saidfurther set of bandpass filter means from location in the path ofincoming radiation and presenting said first, second, third and fourthbandpass means in location of the path of incoming radiation.

13. A radiometer apparatus as claimed in claim 12 and wherein saidindicating means contains a resettable integrator means for samplingsignals corresponding to radiation passed by said first, second, thirdand fourth bandpass filter meansand by said further set of bandpassfilter means.

14. A radiometer apparatus as claimed in claim 13 and furthercomprising:

logic timing means for timing rotations of said filter housing memberand for timing reset of said resettable integrator between statescorresponding to sampling of signals corresponding to radiation passedthrough said first, second, third and fourth bandpass filter means andsampling of 2 signals corresponding to radiation passed through saidfurther set of bandpass filter means.

15. A radiometer apparatus as claimed in claim 14 and wherein said firstbandpass filter means is operable with radia tion in a sub-band having ageneralized absorption coefficient approximately twice that of the bandradiation with which said second bandpass filter means is operable andsaid third bandpass filter means is operable with radiation in asub-band having a generalized absorption coefficient large compared withthat of the sub-band of radiation with which said second bandpass filtermeans is operable.

16. A radiometer apparatus as claimed in claim 14 and wherein one andthe same detector is sensitive to incident radiation in said first,second, third and fourth sub-bands.

17. A radiometer apparatus as claimed in claim 14 and having a certainline of sight usable on an aircraft and further comprising:

means for stabilizing said line of sight against aircraft pitching andmeans for scanning in azimuth said line of sight.

18. A radiometer apparatus as claimed in claim 17 and further comprisinga first housing member for housing the optics of said radiometerapparatus,

a second housing member rotatable within said first housing member aboutan axis perpendicular to said line of sight,

a mirror for deflecting incoming radiation towards said filter means,

a first gyroscopic means, sensitive to aircraft pitching, forcontrolling the angle between said mirror and said axis about which saidsecond housing member is rotatable, and

a second gyroscopic means for rotating said second housing member andscanning in azimuth said line of sight.

19. A radiometer apparatus as claimed in claim 1 and further comprising:

a further set of bandpass filter means in addition to said first,second, third and fourth bandpass filter means,

a filter housing member for housing said first, second, third and fourthbandpass filter means and said further set of bandpass filter means andmeans for rotating said filter housing member and for removing saidfurther set of bandpass filter means from location in the path ofincoming radiation and presenting said first, second, third and fourthbandpass means in location of the path of incoming radiation.

20. A radiometer apparatus as claimed in claim 19 and wherein saidindicating means contains a resettable integrator means for samplingsignals corresponding to radiation passed by said first, second, thirdand fourth bandpass filter means and by said further set of bandpassfilter means.

21. A radiometer apparatus as claimed in claim 20 and furthercomprising:

logic timing means for timing rotations of said filter housing memberand for timing reset of said resettable integrator between statescorresponding to sampling of signals corresponding to radiation passedthrough said first, second, third and fourth bandpass filter means andsampling of signals corresponding to radiation passed through saidfurther set of bandpass filter means.

22. A radiometer apparatus as claimed in claim 21 and wherein said firstbandpass filter means is operable in a subband having a generalizedabsorption coefficient approximately twice that of the sub-band ofradiation with which said second bandpass filter means is operable andsaid third bandpass filter means is operable with radiation in asub-band having a generalized absorption coefficient large compared withthat of the sub-band of radiation with which said second bandpass filtermeans is operable.

23. A radiometer apparatus as claimed in claim 22 and wherein saidfirst, second and third bandpass filter means are operable withradiation from the atmosphere having only wavelengths in sub-bandswithin the carbon dioxide absorption waveband at and ar un d 12 microns.

1. An infrared radiometer apparatus for detecting clear-air turbulencefrom an aircraft comprising: a first bandpass filter means for passingradiation from the atmosphere having only wavelengths in a firstsub-band within an absorption band of a given component of theatmosphere, a second bandpass filter means for passing radiation fromthe atmosphere having only wavelengths in a second sub-band within saidabsorption band, said second sub-band having a different generalizedabsorption coefficient from said first sub-band, a third bandpass filtermeans for passing radiation from the atmosphere having only wavelengthsin third sub-band within said absorption waveband, said third sub-bandhaving a different generalized absorption coefficient from said firstand said second sub-bands, infrared detector means located in the pathof radiation passing through said first, second and third bandpassfilter means and sensitive to radiation passing therethrough forgenerating first, second and third signals corresponding respectively toradiation having wavelengths in said first, second and third sub-bands,and quotient-indicating means responsive to the magnitude of said first,said second and said third signals and indicative of a quotient functionrepresenting the difference between the magnitude of said first signaland the magnitude of said second signal, the difference being divided bythe magnitude of said third signal.
 2. A radiometer apparatus as claimedin claim 1 and wherein one and the same detector is sensitive toincident radiation in said first, said second and said third sub-bands.3. A radiometer apparatus as claimed in claim 1 and wherein said first,second and third bandpass filter means are operable with radiation fromthe atmosphere having only wavelengths in sub-bands within the carbondioxide absorption waveband at and around 15 microns.
 4. A radiometerapparatus as claimed in claim 1 having certain line of sight usable onan aircraft and further comprising means for stabilizing said line ofsight against aircraft pitching, and means for scanning in azimuth saidline of sight.
 5. A radiometer apparatus as claimed in claim 1 andfurther comprising a first housing member for housing the optics of saidradiometer apparatus, a second housing member rotatable within saidfirst housing member about an axis perpendicular to said line of sight,a mirror within said second housing member for deflecting incomingradiation towards said filter means, a first gyroscopic means, sensitiveto aircraft pitching, for controlling the angle between said mirror andsaid axis about which said second housing member is rotatable, and asecond gyroscopic means for rotating said second housing member andscanning in azimuth saId line of sight.
 6. A radiometer apparatus asclaimed in claim 1 and further comprising: a fourth bandpass filtermeans for passing radiation from the atmosphere having only wavelengthsin a fourth sub-band within said absorption waveband, said fourthsub-band having a different generalized absorption coefficient from thatof said first, said second and said third sub-bands, said detector meansbeing sensitive to radiation passing in said fourth sub-band forgenerating a fourth signal corresponding to radiation in said fourthsub-band, and wherein said quotient-indicating means is responsive alsoto the magnitude of said fourth signal corresponding to radiation havingwavelengths in said fourth sub-band and indicative also of a quotientfunction representing the quotient of the difference between themagnitude of said first signal and said second signal and the differencebetween the magnitude of said third signal and said fourth signal.
 7. Aradiometer apparatus as claimed in claim 6 and wherein one and the samedetector is sensitive to incident radiation in said first, said secondand said third sub-bands,
 8. A radiometer apparatus as claimed in claim6 and wherein one and the same detector is sensitive to incidentradiation in said first, second, third and fourth sub-bands.
 9. Aradiometer apparatus as claimed in claim 6 and wherein said first,second, third and fourth bandpass filter means are operable withradiation from the atmosphere having only wavelengths in sub-band withinthe carbon dioxide absorption waveband at and around 15 microns.
 10. Aradiometer apparatus as claimed in claim 6 and having a certain line ofsight usable on an aircraft and further comprising: means forstabilizing said line of sight against aircraft pitching and means forscanning in azimuth said line of sight.
 11. A radiometer apparatus asclaimed in claim 10 and further comprising a first housing member forhousing the optics of said radiometer apparatus, a second housing memberrotatable within said first housing member about an axis perpendicularto said line of sight, a mirror within said second housing member fordeflecting incoming radiation towards said filter means, a firstgyroscopic means, sensitive to aircraft pitching, for controlling theangle between said mirror and said axis about which said second housingmember is rotatable, and a second gyroscopic means for rotating saidsecond housing member and scanning in azimuth said line of sight.
 12. Aradiometer apparatus as claimed in claim 6 and further comprising: afurther set of bandpass filter means in addition to said first, second,third and fourth bandpass filter means, a filter housing member forhousing said first, second, third and fourth bandpass filter means andsaid further set of bandpass filter means, and means for rotating saidfilter housing member and for removing said further set of bandpassfilter means from location in the path of incoming radiation andpresenting said first, second, third and fourth bandpass means inlocation of the path of incoming radiation.
 13. A radiometer apparatusas claimed in claim 12 and wherein said indicating means contains aresettable integrator means for sampling signals corresponding toradiation passed by said first, second, third and fourth bandpass filtermeans and by said further set of bandpass filter means.
 14. A radiometerapparatus as claimed in claim 13 and further comprising: logic timingmeans for timing rotations of said filter housing member and for timingreset of said resettable integrator between states corresponding tosampling of signals corresponding to radiation passed through saidfirst, second, third and fourth bandpass filter means and sampling ofsignals corresponding to radiation passed through said further set ofbandpass filter means.
 15. A radiometer apparatus as claimed in claim 14and wherein said first bandpass filter means is operable with radiatIonin a sub-band having a generalized absorption coefficient approximatelytwice that of the band radiation with which said second bandpass filtermeans is operable and said third bandpass filter means is operable withradiation in a sub-band having a generalized absorption coefficientlarge compared with that of the sub-band of radiation with which saidsecond bandpass filter means is operable.
 16. A radiometer apparatus asclaimed in claim 14 and wherein one and the same detector is sensitiveto incident radiation in said first, second, third and fourth sub-bands.17. A radiometer apparatus as claimed in claim 14 and having a certainline of sight usable on an aircraft and further comprising: means forstabilizing said line of sight against aircraft pitching and means forscanning in azimuth said line of sight.
 18. A radiometer apparatus asclaimed in claim 17 and further comprising a first housing member forhousing the optics of said radiometer apparatus, a second housing memberrotatable within said first housing member about an axis perpendicularto said line of sight, a mirror for deflecting incoming radiationtowards said filter means, a first gyroscopic means, sensitive toaircraft pitching, for controlling the angle between said mirror andsaid axis about which said second housing member is rotatable, and asecond gyroscopic means for rotating said second housing member andscanning in azimuth said line of sight.
 19. A radiometer apparatus asclaimed in claim 1 and further comprising: a further set of bandpassfilter means in addition to said first, second, third and fourthbandpass filter means, a filter housing member for housing said first,second, third and fourth bandpass filter means and said further set ofbandpass filter means and means for rotating said filter housing memberand for removing said further set of bandpass filter means from locationin the path of incoming radiation and presenting said first, second,third and fourth bandpass means in location of the path of incomingradiation.
 20. A radiometer apparatus as claimed in claim 19 and whereinsaid indicating means contains a resettable integrator means forsampling signals corresponding to radiation passed by said first,second, third and fourth bandpass filter means and by said further setof bandpass filter means.
 21. A radiometer apparatus as claimed in claim20 and further comprising: logic timing means for timing rotations ofsaid filter housing member and for timing reset of said resettableintegrator between states corresponding to sampling of signalscorresponding to radiation passed through said first, second, third andfourth bandpass filter means and sampling of signals corresponding toradiation passed through said further set of bandpass filter means. 22.A radiometer apparatus as claimed in claim 21 and wherein said firstbandpass filter means is operable in a sub-band having a generalizedabsorption coefficient approximately twice that of the sub-band ofradiation with which said second bandpass filter means is operable andsaid third bandpass filter means is operable with radiation in asub-band having a generalized absorption coefficient large compared withthat of the sub-band of radiation with which said second bandpass filtermeans is operable.
 23. A radiometer apparatus as claimed in claim 22 andwherein said first, second and third bandpass filter means are operablewith radiation from the atmosphere having only wavelengths in sub-bandswithin the carbon dioxide absorption waveband at and around 15 microns.